Such a plurality of transport cylinders include an impression cylinder opposite to a printing unit and a pair of delivery cylinders positioned at both sides of the impression cylinder in its transport direction for delivering a sheet of paper onto and from the impression cylinder. Each of these cylinders is provided on its peripheral surfaces with grippers whereby sheets of papers are retained as they are uniformly distributed on the peripheral surface while being transported with the cylinder.
In a printing machine using grippers and in which a sheet of paper is transported, by being delivered, from a delivery cylinder onto an impression cylinder and then onto another delivery cylinder, the sheet of paper is delivered and transported while being gripped in turns by such grippers mounted on the peripheral surface of each cylinder. The grippers on each cylinder are mounted upon being allocated circumferentially thereof. A gripper for each cylinder is allocated and mounted thereon at a precision peculiar to its peripheral length, and grippers are incorporated into each cylinder at their mounting position at their individual assembly precisions, respectively.
Thus, where a sheet of paper is being transported from a delivery cylinder onto an impression cylinder and further onto another delivery cylinder while being gripped in turns, depending on mounting positions of the grippers there are introduced a small inherent error in the timing of gripping in turns, which causes an error in each transport of the sheet of paper and in its turn a mutual deviation between images printed.
For this, in a printing machine in which a sheet of paper is transported on a plurality of cylinders with grippers all the cylinders may be made identical in diameter, which may prevent an error in transport due to a gripper from being accumulated while the sheet of paper is in transport over between a plurality of printing units (see, e.g., JP H10-128947 A).
In a printing machine using grippers with which a sheet of paper are transported by being delivered from a delivery cylinder onto an impression cylinder and further onto another delivery cylinder, it is advantageous, for example, to use the impression cylinder having a peripheral length two times greater than a length of the sheet of paper in its transport direction and the delivery cylinder having a peripheral length three times greater than the length of the sheet of paper, and to use two and three grippers mounted respectively on the peripheral surfaces of the impression and delivery cylinders. Gripped, delivered and re-gripped, the sheet of paper in transport when it comes on a surface of the impression cylinder with which a printing unit is juxtaposed is printed on thereby. Then, referring to FIG. 2 where grippers mounted at circumferentially allocated positions on the impression cylinder as a twofold cylinder (a cylinder having two grippers) are assumed to be a and b and grippers mounted at circumferentially allocated positions on the delivery cylinder as a threefold cylinder (a cylinder having three grippers) are assumed to be x, y and z, successive sheets of paper successively delivered from the delivery cylinder onto the impression cylinder are specifically delivered successively between a and x, b and y, a and z, b and x, a and y and b and z. A problem here arises that each allocated position used for delivery may come to be varied for each sheet of paper in transport. It has been difficult to adjust a sheet transport precision by making an individual error adjustment for each gripper.
For this, in a printing machine with an impression and a delivery cylinder having their respective peripheral lengths which are two and three times of the length of a sheet of paper in its transport direction as mentioned above, high-precision machining and high-precision assembly of individual grippers are required. Also, high-precision adjusting devices are necessary. These needs bring about problems of lengthy periods of adjustment time and raised cost of manufacturing equipment
With an impression and a delivery cylinder identical in peripheral length, too, if they are such as two-, three-, four-fold cylinders each provided on its peripheral surface with a plurality of grippers, mutual mechanical register adjustments of these grippers have been time-consuming and hard to make.
In view of the foregoing, it is an object of the present invention to provide a digital sheet-fed printing method that is capable of non-mechanically adjusting and eliminating a mutual deviation between images printed by a plurality of printing units in a digital sheet-fed printing machine using a plurality of transport cylinders.